In English, it needs to be either "a good experience" or "good experiences." Just "good experience" does not work.
I'm not so sure. We can certainly say: 'I have had experience with it'. While I haven't had occasion to replace 'experience' with 'good experience', I have used 'significant experience'.
I'd say 'do you have good experience' or 'are you experienced'. I don't think erfaring is the type of experience you can have in the plural.
"Erfaring" certainly works in plural too. Eg., "det har jeg gode erfaringer med".
Wasn't there another word for experience? Something like oplevelse?
Yes. Erfaring: https://ordnet.dk/ddo/ordbog?query=erfaring